Many hydrocarbon conversions and oxygenate conversions are used in the petrochemical, chemical and refining industries. The conversion processes produce olefins, gasoline, fuel oils, and many other valuable products. Most of the processes are catalyzed by molecular-sieve containing catalysts. Hydrocarbon cracking--a widely practiced hydrocarbon conversion--is an endothermic conversion which commonly is practiced in a fluid catalytic cracking mode (FCC) or in a moving bed cracking mode. Oxygenate conversions are exothermic conversions. Different operating modes, including fixed bed, fluidized bed and moving bed modes, also have been used and tested for oxygenate conversions. Heat must be provided for endothermic processes and must be removed for the exothermic processes. Some processes operate in an adiabatic fashion with little heat added or removed during the reaction.
Both fluidized bed and moving bed processes commonly are carried out in a cyclic mode. The hydrocarbon and oxygenate feedstocks are contacted in a reactor with hot, active catalysts particles at an elevated temperature and a modest pressure. As the feedstocks are converted to the desired valuable products, undesirable residue known as coke forms on the catalyst. The coke tends to cause the catalyst to lose its activity and become deactivated. The deactivated particulate catalysts then are disengaged from the feedstock, sometimes stripped of residual hydrocarbons, and sent to a regenerator for regeneration. Regeneration usually is carried out at elevated temperature with a controlled burn of the coke in the presence of oxygen and a diluent inert gas to minimize unintended temperature surges. In some cases, regeneration is effected by hydrogen stripping at elevated temperature.
The catalysts used in these conversions usually are fine powders with a particle size in the range of from about 20 to 250 microns in diameter, most typically averaging in the range of from about 50 to about 150 microns in diameter. If the catalyst particles are too large in diameter, then the particles will not possess the needed fluidization and other flow properties for fluidized-bed and moving-bed processes. If the particles are too small in diameter, then the particles will be carried out of the reactor by the flowing gas.
In most reactor designs, the catalyst is propelled upwardly through a riser reactor zone where the catalyst contacts a feed. The coked or deactivated catalyst particles are disengaged from the products and any un-reacted feed. After stripping, the catalyst particles are transferred to a regenerator for regeneration. The regenerated catalyst then flows downwardly from the regenerator to the bottom of the riser reactor, and the cycle is repeated.
The cycles of reaction and regeneration are carried out at high temperatures and high flow rates. Collisions and abrasions between catalyst particles themselves, between the particles and reactor walls and between the particles and other parts of the unit tend to cause physical breakdown of the original particles into smaller particles known as fines. This physical breakdown is called attrition. The fines usually have particle diameters smaller than about 20 microns--much smaller than the starting particles. Most commercial reactors are fitted with cyclones to recover the fines, and/or with electrostatic precipitators to prevent the fines from becoming airborne.
Catalysts with higher attrition resistance are desirable because, among other reasons, fewer fines are generated for disposal, less environmental impact is caused by un-recoverable airborne particulates, operating costs are lower, and less catalyst is required due to reduced catalyst consumption.